Field of the Invention
The present invention relates to an image communication apparatus, a control method therefor and a storage medium, and more particularly, to an image communication apparatus using an image communication technique for performing ITU-T Recommendation T.30 in-band facsimile transfer via a high-speed IP network, a control method therefor and a storage medium.
Description of the Related Art
There are two kinds of methods for FAX transmission using a high-speed IP network, which has been spreading recently. One is a transmission method using ITU-T Recommendation T.38 protocol. It is a method in which a signal transmitted by the T.30 protocol used in conventional G3 FAX is transmitted as a digital signal. The T.38 protocol enables higher-speed transmission in comparison with the T.30 protocol.
The other is an in-band transmission method using the ITU-T Recommendation T.30 protocol. This is a method in which data is modulated to an analog signal by a modem, and the analog signal is encoded by a voice codec, further RTP-packetized by an IP header being attached thereto and transmitted. In the in-band transmission method using the T.30 protocol, analog signal is transmitted being deemed to be an audio signal. The method is also called IP/G3 communication because G3 FAX communication is realized by using an IP network.
In an IP phone, a telephone conversation is realized by encoding a voice to PCM data or the like and transmitting/receiving the PCM data on an IP network. By using this technique, it is possible to realize a telephone conversation with a handset/slave phone (hereinafter referred to as a “handset”) of a FAX. By combining the IP/G3 communication and the IP phone, manual transmission by an IP/G3 FAX can be performed.
The manual transmission is a function of performing FAX communication after confirming the situation of a counterpart by a telephone. Basically, a sender makes a conversation directly with the counterpart to be a recipient to mutually confirm an intention of FAX communication with each other, and then the sender starts FAX transmission, and the recipient starts FAX reception. If a recipient apparatus automatically receives FAX, a signal sound made by the recipient apparatus flows through the handset of the sender, thus the sender starts FAX transmission, being triggered thereby.
As a call control means used for the above manual transmission, SIP (Session Initiation Protocol) is used on an IP network, and point-to-point transmission is realized. In the SIP protocol for the call connection, connection is requested by an INVITE message. In the INVITE message, an SDP (Session Descript Protocol) is described. The SDP is a protocol for describing a multimedia session. For example, information about a media type for communication, such as whether a voice is to be communicated (“audio”), an image is to be communicated (“image”) or a video is to be communicated (“video”), is described. The media type is used to identify the purpose of communication. In general, the media type is “audio” in the case of IP phone or IP/G3 communication and is “image” or “application” in the case of T.38 communication.
In the case of performing manual transmission by IP/G3 FAX, the sender makes an IP phone call to a recipient apparatus by a handset first. Here, a transmitting apparatus issues an INVITE message with the media type “audio” and connects an audio session with the recipient apparatus. If the recipient apparatus is a machine compatible with IP/G3 FAX and is in an automatic reception mode, the recipient apparatus sends out a FAX signal such as a CED with the media type “audio”. The sender confirms a FAX signal sound such as a CED sent out from the recipient apparatus through the handset, and presses a FAX transmission start button to start FAX transmission. If the recipient apparatus is a machine compatible with IP/G3 FAX and it is in a manual reception mode, the recipient responds with a handset. Therefore, after the sender and the recipient confirm an intension of FAX communication with each other through the handset, the sender presses down the FAX transmission start button to start FAX transmission.
In these cases, if both of the transmitting apparatus and recipient apparatus are IP FAXs having a T.38 communication function, it is also possible to re-established a session for the media type “image” and perform T.38 communication. In that case, at the timing of pressing down the FAX transmission start button, the transmitting apparatus issues an INVITE message with the media type “image” to the recipient apparatus and attempt switching to T.38 communication. In any of the cases, the timing of pressing down the FAX transmission start button is measured on the basis of an “audio” FAX signal sound or a voice sent from the recipient after a session is established with the media type “audio” (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2007-336161).
However, an IP FAX machine which is not  compatible with IP/G3 communication but compatible only with T.38 communication does not send out a FAX signal sound such as a CED with the media type “audio” upon reception of an INVITE message with the media type “audio”, and immediately sends out an INVITE message with the media type “image” to re-establish a session and attempt T.38 communication. When performing manual transmission to such a recipient apparatus, a sender cannot hear an “audio” FAX signal sound sent out by the recipient apparatus or the recipient's voice, and there is a problem that the sender cannot measure the timing of starting FAX transmission.